As precious metal deposits, particularly gold, become scarcer, mining companies are being forced to exploit refractory precious metal deposits. Typically, gold ores and concentrates are processed using cyanide leaching to dissolve the contained gold. When the cyanide leaching efficiency (i.e., gold recovery) is low, the gold ores and concentrates are called refractory. Often, gold ores/concentrates are refractory because the gold is so finely distributed or as solid solution in a sulphide mineral matrix and/or because of the presence of gold-absorbing carbonaceous materials, and/or because of the presence of cyanicides such as copper oxide and secondary copper sulphide minerals. In refractory sulphide minerals, the gold-bearing sulphides are typically chalcopyrite, pyrite and arsenopyrite. When gold is present as solid solution in a sulphide, no reasonable amount of grinding will liberate the gold from its matrix and make it accessible to cyanide leaching. To render gold sulphide materials amenable to cyanide leaching, the sulphide matrix must be destroyed.
In one method, the sulphide matrix is destroyed through biological oxidation. Sulphide and iron oxidizing microbes (most commonly Thiobaccilus Ferrooxidans and Thiobacillus-Thiooxidans) are used. The microbes are blended in a pulp or a heap with the sulphide minerals. Under bacterial activity, the sulphide minerals are oxidized until the precious metal is freed from the sulphide matrix. The oxidized minerals are then subjected to cyanide leaching to solubilize the gold. The solubilized gold may thereafter be readily recovered by a variety of techniques.
In other methods, the sulphide matrix is destroyed through chemical oxidation. In one chemical oxidation technique, the gold-bearing sulphide minerals are oxidized or calcined or microwaved in a furnace at high temperatures (450-750° C.), in an oxidizing environment. The resulting oxidized product (calcine) can be leached successfully with cyanide. In another chemical oxidation technique, called pressure oxidation, the gold-bearing sulphide minerals are oxidized in an autoclave at high temperature (190-230° C.) and super atmospheric pressure, while injecting oxygen gas through the pulp. For both the bacterial oxidation and pressure oxidation processes, it is necessary to wash for removal of acid and dissolved metals and then neutralize the resulting pulps prior to cyanidation, which is usually carried out at a pH between about pH 9.0 and pH 11.0.
Pressure oxidation reactions for gold bearing sulphide minerals (pyrite FeS2 and arsenopyrite FeAsS) can be written ideally as:4FeS2+15O2+8H2O→2Fe2O3+8H2SO4 and2FeAsS+7O2+6H2O→2FeAsO4.2H2O+2H2SO4 Small amounts of iron and arsenic in the sulphide materials are also converted to the dissolved ferrous iron, ferric iron, arsenite and arsenate. Under these conditions, iron is precipitated in the autoclave as hematite (Fe2O3) and scorodite (FeAsO4.2H2O), and sulphuric acid is generated in solution. These two iron compounds are very desirable because they are chemically stable. It is possible to form other stable Fe—As compounds in the autoclave, depending on the temperature, the Fe/As ratio and the acidity in the autoclave liquor. Because of their chemical stability, these compounds are inert during the subsequent neutralization and cyanidation steps and, therefore, do not consume expensive chemicals, such as lime.
Unfortunately, depending on the chemical conditions prevailing in the autoclave, other less desirable iron compounds can be formed. One such compound is basic iron sulphate, FeOHSO4. Another fairly unstable compound that can form is jarosite. The chemical formula for hydronium jarosite is (H3O)Fe3(SO4)2(OH)6. Other jarosites are also frequently encountered (where the hydronium ion, (H3O)+ is replaced with Na+, K+, NH4+, ½Pb2+, Ag+).
Jarosites and basic iron sulphates can be chemically instable. For example, in the autoclave discharge, basic iron sulphate can react with lime during pre-cyanidation neutralization to form ferric hydroxide and calcium sulphate:FeOHSO4+Ca(OH)2+2H2O═Fe(OH)3+CaSO4.2H2OAlso, some jarosites, particularly hydronium jarosite, react with lime during pre-cyanidation neutralization, to form ferric hydroxide and calcium sulphate:(H3O)Fe3(SO4)2(OH)6+2H2O+2Ca(OH)2→3Fe(OH)3+2CaSO4.2H2O
The instability of basic iron sulphates and jarosites can have a significant economic impact on precious metal operations. When hematite is formed, all the sulphide sulphur in the original autoclave feed ends up as free sulphuric acid and dissolved metal sulphates in solution, and as solid, chemically stable and inert calcium sulphate (if calcite and/or other calcium containing minerals are present in the feed). Therefore, neutralization of the free acid and dissolved sulphate salts in the discharge from the autoclave can be achieved with limestone (CaCO3), which is normally a very cheap reagent. When basic iron sulphate and/or hydronium jarosite is formed during pressure oxidation on the other hand, a significant amount of the basic iron sulphate and jarosite is precipitated and cannot be separated from the precious metal-containing solids. When the solids are neutralized before cyanidation, the basic iron sulphate and jarosite solids are stable in the presence of limestone. The neutralization of basic iron sulphate and jarosite can only be done with stronger but much more expensive neutralization agents, such as lime, CaO, or sodium hydroxide, NaOH, etc. Although jarosite commonly reacts at ambient temperature slowly with such acid neutralizing agents, basic iron sulphate reacts rapidly with the neutralizing agents and can require a large quantity of neutralizing agents to raise the slurry pH to a level suitable for cyanide leaching of precious metals. Therefore, to save on operating costs, it is important to use oxidation conditions disfavoring the formation of basic iron sulphate and to a lesser extent hydronium jarosite and favoring the formation of hematite.
Reaction conditions favoring hematite formation and disfavoring basic iron sulphate and jarosite formation are well known in the literature. For example, higher autoclave slurry temperatures and lower sulphuric acid concentrations favor hematite formation. But the slurry temperature and sulphuric acid concentration of a pressure oxidation process are usually dictated by other constraints (e.g., the rate of sulphide oxidation, the size of the autoclave, the total pressure of the autoclave and the economic requirement for autothermal conditions in the autoclave etc.).
The presence of certain substances is known to affect the formation of basic iron sulphate, jarosite and hematite. While high concentrations of certain cations in the autoclave liquor (in particular monovalent ions such as (H3O)+, NH4+, Na+, K+ and Ag+) normally favor jarosite formation, the presence of divalent metal sulphates in the autoclave liquor (i.e. ZnSO4, CuSO4, MgSO4, MnSO4, etc.), normally favors hematite formation, by lowering the activity of the hydrogen ion. When already present, hematite acts as a seed material that favors continued hematite formation and disfavors basic iron sulphate and jarosite formation.
The factor normally having the greatest impact on the form of the iron species produced in the autoclave is the acidity of the slurry, with high acidity favoring basic iron sulphate and jarosite formation and low acidity favoring hematite formation. Therefore, to form hematite, or even to convert basic iron sulphate or hydronium jarosite to hematite in the autoclave, it is well known that (at a given temperature) acidity control is important.
There are two primary ways to control acidity in the autoclave, namely dilution of the pulp and consumption of some of the acid in the reactor. In the former case, the volume of the pulp is increased while maintaining the number of moles of acid relactively constant[j1]. In the latter case, the volume of the pulp is maintained constant but the number of moles of acid decreased by the addition of neutralizing agents directly to the autoclave.
Dilution is normally effected by adding water to the pulp, thereby lowering the acid concentration and raising the pH. Increasing the dilution of the feed slurry can substantially increase capital costs. Autoclave vessels must be larger for a given ore throughput, and increased dilution also increases the operating costs when dilution of the heat of reaction is excessive and beyond autogenous operations.
Acid consumption can be performed by numerous techniques. It is known to add zinc oxide or any other bases to control acidity and favor the formation of hematite over jarosite. It is known to add limestone (CaCO3) to improve silver recovery, by consuming acid in the autoclave and promoting the formation of hematite over silver jarosite. The recommended limestone addition rates were between 0.50 and 1.67 (CO3/S w/w). It is also known to use ammonia (NH3 or NH4OH) to convert jarosite to hematite, with a molar ratio of NH3/S greater than 2 being preferred. As in the case of dilution, the addition of acid consuming or neutralizing agents increases operating costs because of reagent costs. If the cheapest base limestone (CaCO3) is used, operating costs increase due to carbon dioxide (CO2) evolution in the autoclave, which results in higher venting from the autoclave to remove the CO2 that is formed by the reaction of limestone with sulphuric acid and/or the dissolved metal sulphates. Excessive venting wastes oxygen and upsets the heat balance in the autoclave. It would be desirable to achieve the objective of promoting the formation of hematite over basic iron sulphate and/or hydronium jarosite without incurring a significant increase in capital and/or operating costs.